The method disclosed and claimed herein generally pertains to a magnetic resonance (MR) imaging technique known as diffusion-weighted echo planar imaging (EPI). More particularly, the invention pertains to a method for significantly reducing errors in MR images acquired in accordance with a diffusion-weighted EPI technique, wherein the errors are caused by eddy currents induced by the diffusion-weighting gradient.
As is well known to practitioners in the field of MR imaging, diffusion-weighted images can be obtained using a pair of diffusion-weighting gradients placed, for example, before and after a 180.degree. refocusing radio-frequency (RF) pulse. With the diffusion-weighting gradient, spins with different diffusion coefficients exhibit different degrees of signal loss according to the formula S=S.sub.0 e.sup.-bD. In this expression, S and S.sub.0 are the signals with and without the diffusion-weighting gradient, respectively, D is the diffusion coefficient for a given tissue, and b is known as "b-factor" which is proportional to the square of the diffusion-weighting gradient amplitude. In order for tissues with distinct diffusion coefficients to show adequate contrast in MR images, the amplitude of the diffusion-weighting gradient must be strong enough to ensure a sufficiently large b-factor.
Diffusion-weighted MR imaging technique is particularly useful for early detection of cerebral ischemia. Within only a few hours of the onset of a stroke, diffusion-weighted imaging can highlight the ischemic regions with excellent contrast, whereas other techniques either detect the ischemia at a much later time or cannot detect it at all. Early stroke detection is particularly important as therapeutic drugs such as TPA are effective only within a relatively narrow "therapeutic window", which typically lasts several hours. For effective stroke detection, a b-factor between 600 and 1000 s/(mm).sup.2 is frequently used, resulting in a large diffusion-weighting gradient (e.g., 2 G/cm) to be active for an extended period of time (e.g., 30 ms). Such a diffusion-weighting gradient makes the pulse sequence extremely sensitive to patient motion and eddy current effects. While the motion problem can be effectively removed using the EPI technique, the eddy current problem is aggravated in EPI and causes degrading effects including image shift, geometric distortion, and intensity reduction.